Solid Expandable Tubular Members Formed From Very Low Carbon Steel And Method

ABSTRACT

A very low carbon steel alloy is provided for use in manufacturing tubular members such as oil country tubular goods. The tubular members may be radially expanded from at least twenty percent to forty percent. Sections or joints of casing formed from the steel alloy may be installed within a wellbore and radially expanded during completion of the wellbore.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/440,065, filed May 16, 2003, the entire contents of which areincorporated herein in their entirety by reference.

TECHNICAL FIELD

The present invention is related in general to materials and methodsused to form expandable tubular members and in particular steel alloysand methods for producing oil country tubular goods which may beradially expanded within a wellbore.

BACKGROUND OF THE INVENTION

Wellbores for producing oil, gas or other fluids from subsurfaceformations are often drilled in stages. For example, a wellbore mayfirst be drilled with a drill string and a first drill bit having arelatively large diameter. At a desired depth for a first portion of thewellbore, the drill string and drill bit are removed from the wellbore.Tubular members of smaller diameter, often referred to as casing or acasing string, may then be placed in the first portion of the wellbore.An annulus formed between the inside diameter of the wellbore and theoutside diameter of the casing string is generally filled with cement.The cement provides support for the casing and isolates downholeformations or subterranean strata from each other. Many oil and gaswells are completed with relatively large diameter casing at the wellsurface and smaller diameter casing extending from the large diametercasing in a telescoping or stair step pattern from the well surface to adesired downhole location. One or more strings of production tubingalong with appropriate well completion tools may be installed within thecasing strings for use in producing formation fluids from one or moredownhole locations.

For very deep wells and very long wells, sometimes referred to asextended reach wells (20,000 feet or greater), there may be three orfour changes in casing diameter from the well surface to total depth ofthe wellbore. Each change in casing diameter often results in adecreasing the diameter of production tubing used to produce formationfluids from a desired downhole location. Changes in casing diameterassociated with deep wells and/or long wells result in significantlyincreased drilling and completion costs for associated wells.

Steel, an alloy of iron, is typically made by oxidizing excess carbonand other impurities from molten pig iron. Steel alloys may be producedby injecting substantially pure oxygen into molten iron. Steel alloysmay also be produced in electric furnaces which use iron ore as a sourceof oxygen to remove excess carbon.

Steel alloys typically include relatively high percentages of iron (Fe)and one or more nonmetallic elements. Carbon (C) is one of the mostcommon nonmetallic elements associate with steel alloys. One or moremetal elements in addition to iron may be included in many steel alloys.For example, some steel alloys may contain chromium (Cr) and nickel(Ni). Such alloys may sometimes be referred to as “stainless steel.” Oilcountry tubular goods are frequently formed from steel alloys which havebeen quenched and tempered to produce desired characteristics such asyield strength and ductility. Such steel alloys often have 90% to 95% orgreater tempered martensite by volume of the steel alloy.

Martensite may generally be described as a solid solution of iron whichtypically contains one percent or less of carbon. Martensite is often achief constituent of hardened carbon tool steels. Martensite may beformed by heating steel alloys and then quenching them in cold water.Martensite is sometimes difficult to obtain during quenching of lowcarbon steel alloys and very low carbon steel alloys. A wide variety ofcommercial techniques and procedures have been developed for use insatisfactorily quenching low carbon steel allows and very low carbonsteel alloys with desired martensite concentrations.

A number of oil and gas wells have been completed using solid,expandable casing and other types of solid, expandable tubular members.Electric resistant welded (ERW) pipe has been used to form such casing.Examples of steel alloys and steel compositions which have previouslybeen used to manufacture solid, expandable casing include quenched andtempered steel alloys with carbon concentrations between approximately0.22% and 0.25%. The yield strength of such steel alloys may rangebetween approximately 70,000 and 80,000 pounds per square inch with anupper limit of approximately 95,000 pounds per square inch. Casingformed from such steel alloys may be radially expanded up toapproximately twenty-five percent (25%) within a wellbore. Averageradial expansion for casing formed from such steel alloys may beapproximately fifteen percent (15%).

SUMMARY OF THE INVENTION

In accordance with teachings of the present invention, very low carbonsteel alloys are provided for use in manufacturing solid, expandabletubular members. One aspect of the present invention includes providingthreaded and coupled tubular members which may be releasably engagedwith each other to accommodate radial expansion of the tubular membersat a downhole location during completion of a wellbore. Another aspectof the present invention includes providing tubular members withthreaded swaged ends which may be releasably engaged with each other toaccommodate radial expansion of the tubular members at a downholelocation during completion of a wellbore.

Technical benefits of the present invention include providing steelalloys with very low carbon concentrations satisfactory for use informing solid, expandable tubular members which may be radially expandedfrom approximately twenty percent (20%) to forty-five percent (45%) orgreater. After such radial expansion, the tubular members may stillprovide required mechanical strength and fluid tight integrity forsatisfactory completion of a wellbore and production of formationfluids.

Further technical benefits of the present invention include providingsolid, expandable tubular member formed from very low carbon steelalloys that substantially reduce or eliminate requirements fortelescoping or tapering of wellbores from an associated well surface toa desired downhole location. Such tubular members preferably maintainboth desired mechanical strength and fluid tight integrity during radialexpansion within a wellbore. Expandable tubular members formed inaccordance with teachings of the present invention may allow wells to becompleted to relatively deep geological locations or at extendeddistances from a production platform which may have been difficultand/or expensive to reach using traditional well drilling and casingtechnology. The use of such solid, expandable tubular members may allowwellbores to be drilled and completed with only one or two sizes ofcasing extending from a well surface to a relatively deep downholelocation and/or extended reach location. As a result of requiring onlyone or two sizes of casing to complete a wellbore, surface equipment,associated drilling rigs, drill strings, drill bit sizes and downholewell completion equipment may be standardized to significantly reducecosts.

For some applications tubular members formed in accordance withteachings of the present invention may be radially expanded by as muchas twenty percent (20%) to forty five percent (45%) of their originalinside diameter and satisfactorily hold as much as three thousand fivehundred pounds per square inch (3,500 psi) of internal fluid pressureafter such radial expansion. Tubular members formed from only low carbonsteel alloys in accordance with teachings of the present inventionprovide required mechanical strength to complete deep and/or extendedreach wellbores and provide required fluid pressure tight seals betweenthe interior and the exterior of associated tubular members.

Quench and temper procedures are often limited to use with high carbonsteel alloys and medium carbon steel alloys. Quenching and temperingvery low carbon steel alloys is a relatively unusual procedure.Normalizing is a more common technique associated with very low carbonsteel alloys. Quenching and tempering very low carbon steel alloysformed in accordance with teachings of the present invention may resultin relatively high ductility appropriate for radial expansion ofresulting tubular members in the range of approximately twenty percentto forty-five percent. Quenching and tempering very low carbon steelalloys formed in accordance with teachings of the present inventiontypically produces relatively fine grain structures and relatively highyield strengths associated with oil country tubular goods. Quenching andtempering very low carbon steel alloys formed in accordance withteachings of the present invention results in higher yield strength ascompared with normalizing the same very low carbon steel alloys.Fracture toughness of the resulting tubular members may also beincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 is a block diagram showing one example of a method which may beused to form solid, expandable tubular members from very low carbonsteel alloys in accordance with teachings of the present invention andradially expand such tubular members;

FIG. 2 is a schematic drawing in elevation with portions broken awayshowing an electric resistance welded pipe formed from very low carbonsteel alloys in accordance with teachings of the present invention;

FIG. 3 is a schematic drawing in section and in elevation with portionsbroken away of a tubular member formed from very low carbon steel alloysin accordance with teachings of the present invention having a first,pin end and a second, box end;

FIG. 4 is a schematic drawing in elevation and in section with portionsbroken away showing a first tubular member and a second tubular memberformed from very low carbon steel alloys in accordance with teachings ofthe present invention;

FIG. 5 is a schematic drawing in section showing a coupling formed fromvery low carbon steel alloys in accordance with teachings of the presentinvention; and

FIG. 6 is a schematic drawing in section and in elevation with portionsbroken away showing the coupling of FIG. 5 engaged with a tubular memberformed from very low carbon steel alloys in accordance with teachings ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention and its advantages are bestunderstood by reference to FIGS. 1-6 wherein like numbers refer to sameand like parts.

The term “very low carbon steel alloys” may be used in the steelindustry to describe steel alloys with a concentration of carbon betweenapproximately 0.001% and 0.1% by weight of the steel alloy. Low carbonsteel alloys or mild steel often contains between approximately 0.1% and0.3% carbon. Medium carbon steel alloys may contain betweenapproximately 0.3% and 0.7% carbon. High carbon steel alloys may containbetween approximately 0.7% and 1.5% carbon.

Very low carbon steel alloys formed in accordance with teachings of thepresent invention preferably have carbon concentrations of betweenapproximately 0.03% and 0.06% by weight of the steel alloy. Such verylow carbon steel alloys may also have at least ninety percent (90%) ironby weight of the steel alloy and at least ninety (90%) martensite byvolume of the steel alloy. Often the concentration of iron will be 95%or greater by weight of the very low carbon steel alloy.

The terms “oil country tubular goods” and “OCTG” are used in thisapplication to include casing, tubing, pup joints, couplings and anyother type of pipe or tubular member associated with drilling, producingor servicing oil wells, natural gas wells, geothermal wells or any othersubsurface wellbore.

The terms “welded pipe” and “welded tubular goods” are used in thisapplication to include any pipe, tubular member or coupling manufacturedfrom rolled steel or steel strips which were passed through formingrollers to create a longitudinal butt joint and welded along thelongitudinal butt joint. The resulting longitudinal butt weld orlongitudinal seam weld may be formed using various techniques such aselectric resistance welding (ERW), arc welding, laser welding, highfrequency induction welding and any other techniques satisfactory forproducing longitudinal seam welds. Welded pipe and welded tubular goodsmay be produced in individual links or may be produced in continuouslinks from coiled skelp and subsequently cut into individual links.

The terms “tubular member” and “tubular members” are used in thisapplication to include oil country tubular goods and accessory equipmentsuch as liner hangers, casing nipples, landing nipples and crossconnects associated with completion of oil and gas wells. The terms“tubular member” and “tubular members” are also used in this applicationto include any pipe of any size or any description and is not limited toonly tubular members associated with oil and gas wells.

Various aspects of the present invention will be described with respectto tubular members including couplings which have been formed usingelectric resistant welding (ERW) technology. However, the presentinvention is not limited to use with tubular members produced by ERWtechnology. A wide variety of tubular members including oil countrytubular goods (OCTG) may be formed from very low carbon steel alloysincorporating teachings of the present invention using a wide variety ofwelding techniques.

ERW technology often allows increased quality control of wall thicknessof associated welded pipe and minimizes material defects. Tubularmembers formed in accordance with teachings of the present inventionfrom ERW pipe may have better performance characteristics, such asmechanical strength and fluid tight integrity after radial expansion ascompared with conventional oil country tubular goods formed fromseamless pipe.

FIG. 1 is a block diagram showing one example of a method which may beused to form various types of tubular members including, but not limitedto, oil country tubular goods from very low carbon steel alloysincorporating teachings of the present invention and radially expandingthe resulting tubular members. Method 100 starts at step 102 by forminga very low carbon steel alloys.

For some applications very low carbon steel alloys may be produced in anelectric furnace (not expressly shown). Also, very low carbon steelalloys may be produced by injecting substantially pure oxygen intomolten iron using commercially available equipment and techniques. Othercommercially available techniques associated with manufacturing steelalloys may also be satisfactorily used to produce very low carbon steelalloys incorporating teachings of the present invention. Table A showssome examples of a very low carbon steel alloy formed in accordance withteachings of the present invention.

TABLE A Very Low Carbon Steel Alloys

TABLE A Very Low Carbon Steel Alloys Acceptable Range of Optimum (1)Concentration (1) Concentration Minimum Maximum Carbon (C) 0.045% 0.03%0.06% Manganese (Mn)  1.45% 1.40% 1.50% Phosphorus (P) 0.015%  Sulfur(S) 0.005%  Silicon (Si)  0.23% 0.15% 0.30% Copper (Cu) 0.10% Nickel(Ni) 0.10% Chromium (Cr) 0.10% Molybdenum (Mo) 0.06% Vanadium (V) 0.065%0.05% 0.08% Tin (Sn) 0.01% Aluminum (Al) 0.025% 0.015%  0.040%  Calcium(Ca) 0.0020%  0.0005%  0.0055%  Columbium (Cb)or 0.040% 0.030%  0.050% Niobium (Nb) Boron (B) Res 0.0005%  Max Titanium (Ti) Nitrogen (N)0.010%  MaxNOTES:1. Percentages based on weight of steel alloy.2. Total concentration of V + Nb + Ti limited to 0.15% maximum.3. Liquidus temperature approximately 2770° F.

NOTES:

-   -   1. Percentages based on weight of steel alloy.    -   2. Total concentration of V+Nb+Ti limited to 0.15% maximum.    -   3. Liquidus temperature approximately 2770° F.

At step 104 strips or slabs may be formed from the very low carbon steelalloys using conventional steel fabrication equipment and techniques(not expressly shown).

At step 106 welded pipe may be formed from the steel strips or steelslabs using various techniques including, but not limited to, usingelectric resistance welding. The resulting welded pipe may then bequenched at step 108 to produce at least 90% martensite by volume of theassociated steel alloy. For some applications a high volume water quenchmay be used. U.S. Pat. Nos. 4,417,928 and 4,502,699 show one example ofequipment which may be used to quench welded pipe.

At step 110 the welded pipe may be tempered to produce desired yieldstrength and ductility. An example of welded pipe which has been formedfrom very low carbon steel and quenched and tempered in accordance withteachings of the present invention is shown in FIG. 2.

At step 118 samples may be taken from the steel strips or slabs andanalyzed to determine the specific chemical composition the respectivevery low carbon steel alloy. At step 120 tempering time and/or temperingtemperature for the welded pipe produced in steps 106 and 108 may bemodified based on that chemical composition.

A wide variety of procedures and equipment may be satisfactory used toquench welded pipe at step 108 and temper the welded pipe at step 110.Specific quench and temper procedures will vary depending upon the typeof equipment and manufacturing techniques available at each steelfabrication facility (not expressly shown). Typically, one or morecomputer programs may be empirically derived for each steel fabricationfacility to control associated quench and temper procedures. Quenchingand tempering very low carbon steel alloys at steps 108 and 110 resultsin forming welded pipe with high ductility or high elongationcapabilities, increased toughness with respect to fracture and yieldstrengths satisfactory for use as oil country tubular goods.

Depending upon dimensions such as length, outside diameter and insidediameter, welded pipe formed from very low carbon steel alloys may berapidly quenched using cold water from a temperature of 1650 to 1600° F.to a temperature of 100° F . Based on the chemical composition includingconcentration of carbon in the very low carbon steel alloy, steps 118and 120 may result in a tempering at temperatures of approximately 1200°F. to 1250° F. for approximately 40 minutes to 55 minutes. For very lowcarbon steel alloys with a carbon concentration of approximately 0.045%by weight of this steel alloy, tempering may be conducted atapproximately 1230° F. for approximately 50 minutes.

At step 112 various types of oil country tubular goods may be formedfrom the welded pipe. Examples of such tubular goods include casing 30shown in FIG. 3, coupling 50 such as shown in FIG. 5, and casing 130shown in FIG. 6.

At step 114 the resulting oil country tubular goods may be installedwithin a wellbore (not expressly shown) using well completion equipment(not expressly shown) and techniques associated solid, expandable OCTG.

At step 116 the oil country tubular goods may be radially expandedapproximately 20% to 45% or greater downhole in the wellbore dependingupon overall design of the associated well completion. Such radialexpansion is typically measured by changes in the inside diameter of theoil country tubular goods. Radial expansion may sometimes be conductedin increments such as a first radial expansion of approximately fourteenor fifteen percent. Second and possibly third radial expansions ofapproximately fourteen or fifteen percent may also be performeddepending upon the associated well completion.

Welded pipe 20 formed from a very low carbon steel alloy incorporatingteachings of the present invention is shown in FIG. 2. Welded pipe 20preferably includes first end 21, second end 22 with longitudinal bore24 extending therethrough. As discussed later in more detail, weldedpipe 20 may be used to form tubular member 30 as shown in FIG. 3,coupling 50 as shown in FIG. 5 and/or tubular member 130 as shown inFIG. 6. The length of welded pipe 20 may be substantially varieddepending upon the type of equipment and manufacturing proceduresavailable at each steel fabrication facility. Quenching and temperingprocedures at steps 108 and 110 may be varied depending upon chemicalcomposition and length, outside diameter and inside diameter of weldedpipe 20.

Various aspects of the present invention will be discussed with respectto tubular members 30 as shown in FIGS. 3 and 4. To describe somefeatures of the present invention, tubular members 30 may sometimes bedesignated as 30 a and 30 b. For some applications, each tubular member30 may be a section or joint of a casing string used to complete awellbore (not expressly shown). For such applications, each tubularmember 30 may have overall dimensions and configurations compatible witha conventional oil field casing.

FIG. 3 shows tubular member 30 which may be formed welded pipe 20. Forthis embodiment, tubular member 30 may be generally described as anelongated, hollow section of casing. Tubular member 30 preferablyincludes first pin end 31 and second box end 32 with longitudinal bore24 extending therethrough. Pin end 31 and box end 32 may be formed onrespective first end 21 and second end 22 of welded pipe 20 byconventional “swagging” techniques associated with manufacture of oilcountry tubular goods.

Threaded portions 33 and 34 may be formed on respective pin end 31 andbox end 32 of tubular member 130. Threaded portion 33 and threadedportion 34 may have thread forms or thread profiles similar to AmericanPetroleum Institute (API) buttress threads for oil country tubulargoods. API Specification Standard 5B contains information for varioustypes of threads associated with OCTG. Also, various types of premiumthreads associated with oil country tubular goods may be formed onthreaded portions 33 and 34. Threaded portions 33 and 34 may sometimesbe generally described as modified buttress threads.

For many conventional well completions casing and production tubing aretypically installed in a wellbore with the box end of tubular membersfacing upwards. Most well completion equipment and procedures are basedupon lowering the pin end of a tubular member into engagement with a boxend which is facing upward at the well surface. During completion of awellbore with solid expandable casing such as tubular members 30, it maybe preferable to have pin end 31 of tubular member 30 a facing upwardfor engagement with box end 32 of tubular member 30 b.

Swaged connections such as pin end 31 and box end 32 may provideimproved fluid sealing characteristics during radial expansion of theassociated tubular member within a wellbore. Various types of wellcompletion equipment and techniques may be satisfactorily used toinstall tubular members 30 within a wellbore and to radially expand thetubular members. Depending upon each specific well completion and thetype of radial expansion equipment used to complete each wellbore, theremay be substantial benefits from the perspective of mechanical strengthand/or maintaining fluid tight integrity to use swaged connections suchas pin end 31 and box end 32.

As previously noted, welded pipe 20 as shown in FIG. 2 may be used toform various types of tubular members including couplings 50 as shown inFIGS. 5 and 6. The number of couplings formed from welded pipe 20 willdepend on the length of welded pipe 20 and the desired length of eachcoupling 50. For the embodiment of the present invention as shown inFIGS. 5 and 6 coupling 50 preferably includes first end 51 and secondend 52. Various types of modified buttress threads and/or other threadprofiles may be formed within longitudinal bore 24 of coupling 50. Forthe embodiment shown in FIG. 5 coupling 50 preferably includes threadedportions 61 and 62 which may be generally symmetrically formed relativeto center plane 56 of coupling 50.

Tubular member or casing 130 as shown in FIG. 6 may also be formed fromwelded pipe 20. Tubular member 130 preferably includes first pin end 131and second pin end 132 with longitudinal bore 24 extending therethrough. First pin end 131 and second pin end 132 may be formed onrespective first end 21 and second end 22 of welded pipe 20 usingconventional threading techniques associated with manufacture of oilcountry tubular goods. Threaded portions 133 and 134 may be formed onrespective first pin end 131 and second pin end 132 of tubular member130. Threaded portion 133 and threaded portion 134 preferably havethread forms or thread profiles compatible with threaded portion 61 and62 of casing 50.

For some applications tubular member 130 and associated couplings 50 maybe formed at a oil country tubular good manufacturing facility (notexpressly shown) and engaged with each other as shown in FIG. 6. Theassociated casing section or joint (tubular member 130 and coupling 50)may then be shipped as a unit to a well site for installation within awellbore and radial expansion as previously discussed.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalternations can be made herein without departing from the spirit andscope of the invention as defined by the following claims.

1-10. (canceled)
 11. A tubular member comprising: a welded pipe formedfrom a very low carbon steel alloy having iron at a concentration of atleast ninety-five percent by weight of the steel alloy, chromium at aconcentration of less than approximately 0.1% by weight of the steelalloy, and carbon at a concentration of between approximately 0.03% and0.06% by weight of the steel alloy; the steel alloy quenched to producea concentration of martensite of at least ninety percent by volume ofthe steel alloy; the steel alloy tempered to produce desired ductilityand strength to accommodate radial expansion of the tubular memberbetween approximately twenty percent and forty-five percent; a pin endand a box end formed on the pipe with a longitudinal bore extendingthrough the pipe from the pin end to the box end; an external threadedportion formed on the pin end and an internal threaded portion formedwithin the box end; and the external threaded portion and the internalthreaded portion operable to be releasably engaged with other respectiveinternal and external threaded portions of other tubular members. 12.The tubular member of claim 11 further comprising an inside diameteroperable to be radially expanded at least approximately twenty percentto forty-five percent of the original inside diameter.
 13. The tubularmember of claim 11 wherein the steel alloy further comprises a totalconcentration of vanadium, niobium and titanium limited to less thanapproximately 0.15% by weight of the steel alloy.
 14. The tubular memberof claim 11 further comprising the external threaded portion and theinternal threaded portion having matching modified buttress threadforms.
 15. A solid, expandable casing string for using in completing awellbore comprising: a first tubular member formed from an electricresistance welded pipe; a second tubular member formed from an electricresistant welded pipe; the electric resistance pipes formed from a steelalloy having carbon at a concentration of between approximately 0.03%and 0.06% by weight of the steel alloy; the first tubular member havinga pin end and a box end with a longitudinal bore extending through thefirst tubular member between the pin end and the box end; a firstthreaded portion formed on the pin end of the first tubular member and asecond threaded portion formed in the box end of the first tubularmember; the second tubular member having a pin end and a box end with alongitudinal bore extending through the second tubular member betweenthe pin end and the box end; a first threaded portion formed on the pinend of each tubular member and a second threaded portion formed in thebox end of each tubular member; and the first threaded portion of thepin end of the first tubular member releasably engaged with the secondthreaded portion of the box end of the second tubular member.
 16. Thecasing string of claim 15 further comprising: the steel alloy having lowconcentrations of vanadium, niobium and titanium; and the totalconcentration of vanadium, niobium and titanium less than approximately0.15% by weight of the steel alloy.
 17. A solid, expandable casingstring for using in completing a wellbore comprising: a first tubularmember formed from an electric resistance welded pipe; a second tubularmember formed from an electric resistant welded pipe; the electricresistance welded pipes formed from a steel alloy having carbon at aconcentration of between approximately 0.03% and 0.06% by weight of thesteel alloy; the first tubular member having a first end and a secondend with a longitudinal bore extending through the first tubular memberbetween the first end and the second end; the second tubular memberhaving a first end and a second end with a longitudinal bore extendingthrough the second tubular member between the first end and the secondend; and the first end of the first tubular member releasably engagedwith the second end of the second tubular member;
 18. The casing stringof claim 17 further wherein the steel alloy further comprises; lowconcentrations of vanadium, niobium and titanium; and the totalconcentration of vanadium, niobium and titanium less than approximately0.15% by weight of the steel alloy.
 19. A solid, expandable couplinghaving high ductility satisfactory for completing a wellbore comprising:a first end and a second end with a longitudinal bore extending throughthe coupling between the first end and the second end; the couplingformed from a steel alloy; the steel alloy having carbon at aconcentration of between approximately 0.03% and 0.06% by weight of thesteel alloy; a first threaded portion formed within the longitudinalbore of the coupling; and a second threaded portion formed within thelongitudinal bore of the coupling.
 20. The coupling of claim 19 whereinthe steel alloy further comprises: low concentrations of vanadium,niobium and titanium; and the total concentration of vanadium, niobiumand titanium less than approximately 0.15% by weight of the steel alloy.21-26. (canceled)